Such an attachment pylon is in fact provided to form the connecting interface between an engine and a wing of the aircraft. It makes it possible to transmit, to the structure of said aircraft, the forces generated by its associated engine, and also allows the conveyance of the fuel, the electrical, hydraulic and air systems between the engine and the aircraft.
FIG. 1 shows an engine assembly for an aircraft, as known from document WO 2009/037267. This engine assembly 1 is intended to be fastened under a wing 2 of said aircraft. It includes an attachment pylon or device 4, as well as an engine 6 such as a turbojet engine attached under said device 4.
Overall, the attachment device 4 includes a rigid structure 8, also called primary structure, supporting attachment means for the engine 6, these attachment means having a plurality of engine attachments 10, 12, as well as a device for taking up thrust forces 14 generated by the engine 6.
For information, it should be noted that the assembly 1 is intended to be surrounded by a nacelle (not shown), and that the attachment pylon 4 includes another series of fasteners (not shown) fastened on the rigid structure 8 and making it possible to suspend said assembly 1 under the wing 2 of the aircraft.
In the rest of the description that follows, by convention, X refers to the longitudinal direction of the pylon 4, which can also be likened to the longitudinal direction of the turbojet engine, said direction X being parallel to a longitudinal axis 5 of said turbojet engine 6. On the other hand, Y refers to the direction oriented transversely relative to the pylon 4 and also comparable to the transverse direction of the turbojet engine 6, and Z the vertical direction of the height, these three directions X, Y and Z being orthogonal relative to each other.
The terms “front” and “back” should be considered relative to a direction of travel of the aircraft encountered after the thrust exerted by the turbojet engine 6, said direction being shown diagrammatically by arrow 7.
FIG. 1, showing the prior art, shows the two engine attachments 10, 12, the take-up device for thrust forces 14, the rigid structure 8 of the attachment device 4, as well as a plurality of secondary structures fastened on the rigid structure 8. These secondary structures ensuring the segregation and maintenance of the systems while supporting aerodynamic fairing elements will be described below.
It is indicated that the turbojet engine 6 has, at the front, a fan case 18 with large dimensions delimiting an annular fan channel 20, and has, toward the back, a central case 22 with smaller dimensions, containing the core of said turbojet engine. The cases 18 and 22 are of course secured to each other.
As shown in FIG. 1, two engine attachments 10, 12 of the device 4 are provided, and are respectively called forward engine attachment and aft engine attachment. The forward engine attachment 10 is inserted between a forward end of the rigid structure 8, and an upper part of the fan case 18. The aft engine attachment 12 is inserted between the rigid structure 8 and the central case 22, or the gas exhaust case situated further back.
The rigid structure 8 makes it possible to convey forces between the turbojet engine and the wing system. It assumes the form of a box extending from back to front, substantially in direction X. This metal box is then traditionally formed by assembling upper and lower longerons and side panels connected to each other via transverse inner stiffening ribs (not visible in FIG. 1), which each assume the form of a rectangle oriented in plane YZ.
Still in reference to FIG. 1, the secondary structures of the pylon 4 include a forward aerodynamic structure 24, an aft aerodynamic structure 26, a connecting fairing 28 of the forward and aft aerodynamic structures, and a lower aft aerodynamic fairing 30, also called “shield” or “APF” (Aft Pylon Fairing).
More precisely, the forward aerodynamic structure 24 is placed in the lower forward extension of the wing 2 and above the primary structure 8. It is fixedly mounted on the rigid structure 8, and has an aerodynamic profile function between an upper part of the fan cowls hinged thereon, and the leading edge of the wing system. This forward aerodynamic structure 24 then has not only an aerodynamic fairing function, but also makes it possible to place, segregate and convey different systems (air, electric, hydraulic, fuel). Furthermore, the forward part of this structure 24 not being in contact with the rigid structure 8, a heat exchanger is usually inserted in the space defined between these two elements.
Directly in the rear extension of said structure 24, still under the wing system and mounted above the rigid structure 8, is a connecting fairing 28, also called “karman.” Then, still toward the rear, the connecting fairing 28 is extended by the aft aerodynamic structure 26, which contains part of the equipment of the pylon. This structure 26 is preferably situated completely behind relative to the rigid structure 8, and is therefore attached under the wing of the aircraft.
Lastly, under the rigid structure 8 and the aft aerodynamic structure 26 is the lower aft aerodynamic fairing 30, also called “shield” or “Aft Pylon Fairing.” Its essential functions are the formation of a thermal barrier, also called firewall, serving to protect the pylon and the wing system from the heat given off by the primary flow, and the formation of an aerodynamic continuity between the output of the engine and the attachment pylon. In a known manner, the aforementioned fairing 30 includes a heat protection floor 32 provided with an outer surface intended to be hugged by a primary flow of the engine that it partially delimits, radially outwardly, this primary flow escaping the hose 33 of the engine being diagrammatically shown by arrow 36. Moreover, the fairing 30 also includes two side panels 44, which are provided to be outwardly hugged by a secondary flow from the engine diagrammatically shown by arrow 38, due to their installation in the annular secondary flow channel 40 of the engine, and/or as output therefrom.
It should be noted that in the described preferred embodiment where the engine 6 is intended to be suspended under the wing system of the aircraft, the heat protection floor 32 for protecting the pylon and the wing system from the primary flow 36 forms a lower portion of the fairing 30. Naturally, this floor constitutes an upper portion of the fairing in the alternative case where the engine is intended to be installed above the wing system.
Lastly, as shown in FIG. 1, it is provided that the forward end of the floor 32 hugs the upper aft end of the hose 33, or it is brought much closer to said same aft end of the hose 33.
As mentioned above, the pylon is equipped with a plurality of wing system attachments constituting attachment means for attaching the rigid structure on the wing. One embodiment of these attachment means is disclosed in document FR 2 887 522. They generally include two forward attachments each taking up forces in direction Z, an intermediate attachment called a spigot attachment, taking up forces in directions X and Y, and an aft attachment taking up forces in directions Z and Y.
Although the solution described above enables a satisfactory transmission of the static and dynamic forces created by the engine under all flight conditions, it nevertheless has non-negligible drawbacks.
In fact, on the turboshaft engines of recent aircrafts, the high dilution level sought leads to obtaining an extremely high bulk, since increasing the dilution level inevitably leads to an increase in the diameter of the engine, and more particularly an increase in the diameter of its fan case.
Thus, with a ground clearance that is naturally set so as to remain acceptable from a safety perspective, the space remaining between the wing and the turboshaft engine serving to house the rigid structure of the attachment pylon as well as its different attachments is more and more limited, while paradoxically, the forces to be taken up are of course increasingly high.
This evolution of turboshaft engines has had the harmful consequence of imposing a reduction of the vertical dimensions of the rigid structure of the attachment pylon, in particular so as to be able to keep sufficient space to place the component elements of the intermediate attachment, the large dimensions of which are imposed by the need to take up thrust forces from the turboshaft engine, i.e. those oriented in the longitudinal direction of said turboshaft engine.
Furthermore, providing four distinct wing system attachments inevitably leads to a high overall mass for the attachment pylon.